1) Field of the Invention
This invention relates to a bonding material for an IC or LSI and a lead frame, that is, an electrically conductive bonding film useful as a die bonding material. Further, this invention is concerned with a production process of the electrically conductive bonding film and also with a bonding method making use of the electrically conductive bonding film. The term "electrically conductive" will hereinafter be referred to simply as "conductive" for the sake of brevity, so that the term "conductive" as used herein means "electrically conductive" unless otherwise specifically indicated.
2) Description of the Related Art
Conventionally, Au-Si eutectic alloy, solder, silver paste or the like has been used to bond an IC or LSI with a lead frame.
Au-Si eutectic alloy has high heat resistance and moisture resistance but, for its high modulus of elasticity, is accompanied by the problems that it is prone to breakage When applied to a large chip and is costly.
Solder is inexpensive but is inferior in heat resistance. Further, its modulus of elasticity is high like Au-Si eutectic alloy, thereby making it difficult to apply it to large chips.
Silver paste, on the other hand, is economical and has high moisture resistance. Its modulus of elasticity is the lowest among the three bonding materials described above. It also has heat resistance sufficient to permit application of 350.degree. C. thermocompression wire bonding, so that silver paste is now a primary bonding material for ICs or LSIs and lead frames.
Keeping step with the high densification of an IC or LSI in recent years, chips are becoming larger. Upon bonding an IC or LSI on a lead frame with silver paste, the above trend has led to difficulties in spreading and coating the silver paste on the entire surface of the chip.
In "MICROELECTRONIC MANUFACTURING AND TESTING", October, 1985, there is reported a conductive bonding film which contains a conductive filler filled in a thermoplastic resin and is useful in die bonding. This conductive bonding film is heated approximately to the melting point of the thermoplastic resin and is then compressed to achieve bonding.
In the conductive bonding film reported in the above magazine, selection and use of a thermoplastic resin having a low melting point make it possible to lower the bonding temperature. This can reduce damage to chips such as oxidation of a lead frame. However, its bonding force under heat is low so that it cannot withstand heat treatments after die bonding, such as wire bonding and packaging process. Use of a thermoplastic resin having a melting point high enough to withstand such heat treatments however results in a high bonding temperature. This has led to the potential problem that chips may be damaged such as oxidation of a lead frame.